Many applications require signals, in particular radio frequency signals, that are produced or processed in one circuit part to be provided to another circuit part of a circuit for further use. By way of example, in some applications, signals produced by an oscillator such as a Voltage Controlled Oscillator (VCO) need to be supplied to a frequency divider chain, a transmission circuit or a reception circuit, signals from a frequency doubler need to be supplied to further circuit parts, or part of an output from a transmission circuit needs to be supplied to a diagnosis circuit such as a power detector for test purposes.
For such coupling out of signals from one circuit and coupling in another circuit, it is in many cases desirable or necessary for the operation of the circuit part from which the signal (or multiple signals) is coupled out (subsequently also referred to as the out-coupling circuit part) to be influenced as little as possible. In particular, it is desirable in the case of many applications for such coupling out of a signal to provide as small a load as possible for the out-coupling circuit part.
This is critical particularly when there are many applications in which high frequencies, for example corresponding to millimeter waves (in the range of greater than GHz, for example), are used. An example that may be cited in this case is a voltage controlled oscillator, which uses a resonator tank. When a load is connected directly to nodes of the resonator tank, this loads the resonator tank and influences the phase noise.
In addition, for some applications, it is desirable to couple out a signal not only at one frequency but rather to couple out signals at different frequencies separately from one another from a single circuit. An example may be the coupling out of a higher harmonic of the fundamental signal of an oscillator. If a voltage controlled oscillator operates at a fundamental signal of 30 GHz, for example, then some implementations have a second harmonic at 60 GHz at some nodes. In some oscillator configurations, for example in what is known as a push-push configuration, the fundamental signal (e.g. 30 GHz) is then supplied to a frequency divider chain, and the second harmonic (e.g. 60 GHz) is supplied to a receiver or transmitter. Two frequencies can also occur simultaneously for frequency doublers, circulators or duplexers.
Conventionally, such coupling out of multiple signals at different frequencies is effected by direct coupling, the load being decreased by using smaller transistors, for example, in order to decrease a (parasitic) load capacitance. Nevertheless, this may result in impairments to the circuit from which the coupling out is effected. In addition, such approaches can give rise to additional parasitic effects. Finally, reduction of structures such as transistors also has its limits.